Lignocellulosic composites are conventionally manufactured by hot pressing lignocellulosic materials with wax and thermosetting resin. This is referred to as a conventional bonding process. The wax is a sizing agent to improve the water resistance of the once-formed composite. The resin is a bonding agent that holds the materials comprising the composite together, thus forming them into a unitary shape. Resoles are commonly used as the binding resin for lignocellulosic composites.
In the conventional hot press method of manufacture of lignocellulosic composites, a lignocellulosic material is combined with a phenolic resin and other components in a blender or mixer. The blend or mixture that results is pressed, typically under pressures above atmospheric and temperatures greater than room temperature, to produce the composite. Lignocellulosic materials used in the production of mats may be selected from the group consisting of wood fiber, wood flake, wood strands, wood chips and wood particles, and mixtures thereof. The lignocellulosic materials listed here are referred to in the art as wood furnish. However, it is well known that other wood furnish, such as straw, bagasse, wood bark, recycled wood fiber, recycled paper fiber, and mixtures thereof, may also be used. The wood furnish, once blended or mixed with the phenolic resin, is then formed onto a support material to make a pre-form in the approximate shape of the finished good. The pre-form is then placed on a caul plater in a hot press where the finished good is produced by applying pressures above atmospheric and temperatures greater than room temperature. The elevated temperatures and pressures cause the phenolic resin to polymerize, thus biding the pre-form into a unitary finished good. The hot press method is further described in U.S. Pat. No. 4,433,120 to Shui-Tung Chiu.
Lignocellulosic composites primarily find use in construction or fabrication. These composites may be used in building construction or any fabrication where wood is a traditional material used. The poor dimensional stability of state-of-the-art lignocellulosic composites affects their mechanical properties and reduces their load carrying ability. Another result of poor dimensional stability is unevenness of roof and floor underlayments, and of building siding. Two methods have been principally suggested as means to produce dimensionally stable lignocellulosic composites. However, both of these methods have proven to be too costly to be used in practice. The first method is referred to as Bulking Treatment. In this method, lignocellulosic materials are impregnated with water soluble polymers such as polyethylene glycol or impregnated with a low molecular weight resin such as phenol-formaldehyde or vinyl monomers and polymerized in situ. The second method is referred to as Chemical Modification. In this method, the lignocellulose may be esterified by, for example, acetylation, or it may be cross-linked using, for example, an aldehyde. An alternative method of Chemical Modification is to react hemicellulose with lignin under elevated temperatures, typically using steam treatment. Any of these methods of Chemical Modification, in addition to being costly, also result in reduced strength of the once-formed composite.
A method widely used in the conventional bonding process to improve dimensional stability, as noted above, is the application of a wax sizing agent. The wax sizing imparts a certain degree of water repellency to the once-formed composite. Paraffin is a compound sizing agent. One method by which wax sizing impart water repellency is by coating the surface of the lignocellulose, thus decreasing its surface tension. Another method by which wax sizing imparts water repellency is that wax will partially fill the capillaries within the lignocellulose, thus providing a barrier to the capillary uptake of water.
The phenol-formaldehyde resin used in the manufacture of lignocellulosic composites may be in the form of a solid or a liquid. Powdered phenolic resins, such as novolac, resole, or combinations thereof, may generally be used. U.S. Pat No. 4,098,770 to Berchem, et al., discloses a typical spray-dried phenol-formaldehyde resin, modified with added non-phenolic polyhydroxy compounds, used in the manufacture of waferboard. Liquid phenol-formaldehyde resins, such as resole or resole and novolac combinations, may also be generally used in the manufacture of lignocellulosic composites. Parameters for the manufacture of either liquid or solid phenol-formaldehyde resins are disclosed in Phenolic Resins, Chemistry, Applications and Performance, (A. Knop and I. A. Pilato, Springer-Verlag (1985)) and Advance Wood Adhesives Technology, (A Pizzi, Marcel Deldcer (1994)).
There is a need for lignocellulosic composites that are dimensionally stable when exposed to moisture. There is a further need for lignocellulosic composites that do not swell when immersed in water and that do not shrink when dried.